Reproductive and pregnancy complications are prevalent, are often recurrent and have poorly understood etiologies. There is mounting evidence that innate immune factors are important for reproductive immune homeostasis to support fertilization, implantation, placentation and overall pregnancy health while maintaining protection against pathogens. This is significant as sexually transmitted infections (STIs), which lead to adverse reproductive and pregnancy outcomes, are currently at an exceptional high in the United States. Furthermore, infection-related pregnancy outcomes, such as preterm birth, occur at the same rate in the U.S. as some middle and low- income countries (11%), particularly in minority populations. Even in the absence of STIs, the genital microbiome may have a significant influence on reproductive immunology. Thus, immune molecules may be targets for novel therapies to improve reproductive and pregnancy health. Recently characterized type I interferon epsilon (IFNe) has been reported in animal and experimental models to maintain reproductive tract IFN-stimulated gene expression and form an innate immune defense against STIs. Indeed, studies suggest that IFNe is a potential novel mucosal therapeutic against genital infections. However, the role of IFNe in human reproductive success is not elucidated. There is a critical need to increase the understanding of the epidemiology, biology and clinical utility of IFNe. Pregnancy is a dynamic immunological state and other type I IFNs, such as IFN-beta, modulate maternal immunity, promote tolerance to the fetus and protect against pathogens. However, type I IFNs can also exacerbate disease, particularly following bacterial infections. This is likely due to increased signaling by the innate immune receptors that regulate IFN expression. However, IFNe is unique as it is regulated by reproductive hormones which makes this type I IFN a more desirable target for mucosal therapeutics. A pilot investigation of 25 pregnant women showed that IFNe is present in the lower genital tract, increases as gestation progresses and is lower in pregnant women with genital infections and high body mass index. The objective of this R21 proposal is to 1) validate prior estimates of vaginal IFNe expression across pregnancy; 2) identify maternal demographic characteristics that influence IFNe levels and 3) determine if lower IFNe levels are associated with maternal and infant health indicators. This study is innovative as it will be the first longitudinal investigation of human IFNe in pregnancy. Data generated from this study will be used for future investigations to determine if IFNe has any clinical utility in protection against genital tract infections and improves reproductive and pregnancy success.